In several communications systems the data to be transmitted is compressed so that the available bandwidth is used more efficiently. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172 and is incorporated herein by reference. The second, known as MPEG-2, refers to ISO/IEC standards 13818 and is incorporated herein by reference. A compressed digital video system is described in the Advanced Television Systems Committee (ATSC) digital television standard document A/53, and is incorporated herein by reference.
The above-referenced standards describe data processing and manipulation techniques that are well suited to the compression and delivery of video, audio and other information using fixed or variable length code in digital communications systems. In particular, the above-referenced standards, and other "MPEG-like" standards and techniques, compress, illustratively, video information using intra-frame coding techniques (such as run-length coding, Huffman coding and the like) and inter-frame coding techniques (such as forward and backward predictive coding, motion compensation and the like). Specifically, in the case of video processing systems, MPEG and MPEG-like video processing systems are characterized by prediction-based compression encoding of video frames with or without intra- and/or inter-frame motion compensation encoding.
It is known to compress (i.e., resize) image information to reduce decoder anchor frame memory requirements or to reduce decoder processing resources in, e.g., television systems utilizing relatively low resolution display devices. Such an application is the case of a high definition television (HDTV) receiver associated with a standard definition television (SDTV) display, or providing video information to a conventional NTSC, PAL or SECAM television.
A first known technique comprises decoding at the full HDTV resolution, storing the resulting full resolution pictures and perform filtering and down-sampling on the full resolution pictures before display. While this approach is very flexible in terms of resolutions supported, the cost is prohibitive since the frame-store memory must accommodate the full-resolution pictures. Even if filtering and down-sampling is performed prior to anchor frame storage, the computation complexity is the same as full resolution decoding.
A second known technique comprises, in the case of, e.g., 8.times.8 blocks of DCT coefficients received by an MPEG-like decoder, processing only the 4.times.4 lower (in terms of horizontal and vertical spatial resolution) sub-block of a DCT coefficient block (i.e., truncate the three 4.times.4 higher order sub-blocks). An inverse DCT operation performed on the lower 4.times.4 DCT coefficient block yields only a 4.times.4 pixel block. Both the IDCT computation complexity and the memory requirement for frame stores are reduced in this case.
A third technique is described in a paper published by Bao et al. (J. Bao, H. Sun and T. Poon, "HDTV Down-Conversion Decoder," IEEE Transactions on Consumer Electronics, Vol. 42, No. 3, August 1996) and incorporated herein by reference in its entirety. Specifically, the Bao technique processes, using a frequency synthesis technique, four adjacent 8.times.8 DCT coefficient blocks to produce a new 8.times.8 DCT coefficient block, which is then subjected to an inverse DCT processing to produce an 8.times.8 pixel block. In this manner both the IDCT computation complexity and the memory requirement for frame stores are reduced, with fewer visual artifacts than produced using the second technique described above.
Unfortunately, all of the above-described down-sampling decoders utilize a significant amount of computational resources to implement the inverse DCT function. Therefore, it is seen to be desirable to provide a down-sampling video image decoder providing for at least a greatly reduced inverse DCT resource.